The effect of hydropower on fish stocks: comparison between ...

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32 Hydrobiologia (2008) 609:25–36 Fig. 13 Mean average weight <strong>of</strong> fish in different areas <strong>of</strong> Vrané Reservoir. Significant differences were found between areas. Error bars represents standard deviations (SD) Fig. 11 Temperature and dissolved oxygen pr<strong>of</strong>iles in the cascade Slapy Reservoir. <strong>The</strong> reservoir was stratified in the upper (grey), middle and dam (white) areas and was not stratified in the tributary area (black) are common phenomena in non-cascade reservoirs (Nakashima and Leggett, 1975; Fernandez-Rosado et al., 1994; Hejzlar & Vyhnálek, 1998; Seďa and Deveter, 2000; Fernandez-Rosado & Lucena 2001). Fish distribution reflects these gradients as described by the results <strong>of</strong> this study as well as by other authors (Siller et al., 1986; Brosse et al., 1999; Gido et al., 2002b; Matthews et al., 2004; Vašek et al., 2004). A positive correlation between fish density and the amount <strong>of</strong> total phosphorus or chlorophyll a has been observed in some Scandinavian lakes (Jeppesen et al., 2000; Olin et al., 2002) and German lakes (Mehner et al., 2005). Lakes with high concentrations <strong>of</strong> total phosphorus have a high biomass <strong>of</strong> fish and the proportion <strong>of</strong> cyprinids is also high. It is not possible to distinguish fish species by acoustics but Czech non-cascade reservoirs are dominated by cyprinids (mainly bream (Abramis brama (L.)), roach (Rutilus rutilus (L.)) and bleak (Alburnus alburnus (L.))—Vašek et al., 2004; Prchalová et al., 2006). High density and biomass <strong>of</strong> fish in the tributary area is composed <strong>of</strong> a higher proportion <strong>of</strong> small fish, probably juveniles. Vašek et al. (2004, 2006) observed a higher proportion <strong>of</strong> young-<strong>of</strong>-year (YOY) fish in the tributary <strong>of</strong> Římov reservoir. In the concept <strong>of</strong> riverine origin <strong>of</strong> fish in reservoirs (Fernando & Holčík, 1991; Holčík, 1998) the tributary zone serves as an important spawning area for reservoir fish (Hladík & Kubečka, 2003). Vertical Fig. 12 Observed mean biomass (a) and density (b) <strong>of</strong> fish in the cascade Vrané Reservoir revealed by horizontal acoustics. Significant differences were found between areas. Sazava is the 2nd tributary (see in text). Error bars represents standard deviations (SD) 123
Hydrobiologia (2008) 609:25–36 33 Fig. 14 Temperature and dissolved oxygen pr<strong>of</strong>iles in the cascade Vrané Reservoir. <strong>The</strong> reservoir was stratified in Sázava river arm (grey) and dam area (white) but was not stratified in Vltava river arm (black) distribution also follows a clear pattern due to strong summer stratification. <strong>The</strong> majority <strong>of</strong> the fish biomass was located in surface layers above the thermocline (Schael et al., 1995; Kubečka & Wittingerová, 1998; Vašek et al., 2004). It seems that fish are very unwilling to swim into the colder, less oxygen saturated water below the thermocline. <strong>The</strong> first common feature <strong>of</strong> fish distribution in the Vltava river cascade reservoirs is that the tributary usually appears to be little inhabited by fish compared to the high biomasses and densities in the tributaries <strong>of</strong> non-cascade reservoirs. <strong>The</strong> main reason for the low biomass and density <strong>of</strong> fish seems to be the quality <strong>of</strong> water flowing into the reservoir. In the Vltava River cascade the reservoirs are situated one immediately after another and the water that is discharged into a lower reservoir usually comes from the deep layers <strong>of</strong> the reservoir above, through the turbines (Fig. 1). This hypolimnetic inflow is cold and very poorly saturated with oxygen in summer which is very opposite to the tributary <strong>of</strong> non-cascade reservoirs (Figs. 8, 11 and Hrbáček & Straškraba, 1966; Hrbáček, 1984). Fish distribution in Vrané Reservoir fully supports this hypothesis. Two comparable rivers (in terms <strong>of</strong> average discharge) flow into this reservoir but they differ in temperature and concentration <strong>of</strong> oxygen. A low biomass and density <strong>of</strong> fish was observed in the cold and poorly oxygen saturated Vltava arm. On the other hand, a high biomass and density were observed in the warm and well oxygen saturated Sázava arm as is common in the tributary <strong>of</strong> non-cascade reservoirs. Also, fish biomass and density remain low to the confluent <strong>of</strong> Vltava and Sázava rivers. Releasing <strong>of</strong> cold and low oxygen saturated water from the hypolimnium <strong>of</strong> the upstream reservoir lowers the fish biomass and density in cascade reservoirs. Mass fish kills were even experienced during the summer caused by oxygen deficits. Oxidizing <strong>of</strong> released water from an upstream reservoir might be helpful especially in summer season. A second shared feature <strong>of</strong> fish distribution in the Vltava River cascade reservoirs is the high biomass and density found in dam areas. This biomass was formed mainly <strong>of</strong> adult fish (mean average weight [400 g), especially in Kamýk and Vrané reservoirs. Most <strong>of</strong> these big adult individuals are unlikely to be <strong>of</strong> autochthonous origin. Many carp (Cyprinus carpio) have been flushed into the reservoirs from fish ponds in the catchment during an extreme flooding event in 2002 (Rutkayová et al., 2006) or come from a regular stocking programme <strong>of</strong> the Czech Fishing Union (Draštík et al., 2004) and were using the warmest near-surface layers <strong>of</strong> the dam area. In cascade reservoirs with low reproduction <strong>of</strong> cyprinid fish (Draštík et al., 2004), the input <strong>of</strong> fish by flooding may have a significant impact on increasing standing stock. In non-cascade reservoirs even extreme flooding does not necessarily have a significant impact <strong>of</strong> on the total fish stock (Vašek et al., 2004) or even on fish composition (Gido et al., 2000; Kubečka et al., 2004). <strong>The</strong> development and stability <strong>of</strong> summer stratification was examined by Straškraba & Hocking (2002) who found that retention time (RT) has an important influence on hydrodynamic conditions in a reservoir and on the development and stability <strong>of</strong> 123
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